Automatic frequency control circuit



Sept. 6, 1938. D. E. FOSTER 2,129,123

l AUTOMATIC FREQUENCY CONTROL CIRCUIT Filed March 51, 19:57

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v Y u "u Q 'Bl E U I.' x L: S j dx Nl @im "g" 2mm l es@ mmm i l Hl N n 5: t I Nl l `N` A "l MMM U4 g D Q1 E' E b INVENTOR DUDLEY E. FOSTER ATTORNEY Patented Sept. 6, 1938 raraa'r orifice AUTOMATIC FREQUENCY CONTROL CIRC Dudley E. Foster, South Radio Corporation of Delaware Application March 31,

' '7 Claims.

My present invention relates to automatic frequency control circuits for radio receivers of the superheterodyne type, and more particularly to a device which is adapted to regulate the effect of the frequency control tube on the oscillator tank circuit.

As is Well known at the present time automatic frequency control circuits for superheterodyne receivers generally comprise a discriminator unit for deriving a direct current voltage from the intermediate frequency (I. F.) energy when the latter shifts in frequency from the assigned I. F. value; a frequency control tube network being electrically associated with the local oscillator tank circuit in such a manner as to simulate across the tank circuit a reactance of a predetermined sign; and the voltage output of the discriminator being employed to regulate the magnitude of the simulated reactance across the oscillator tank circuit. In the operation of such an AFC (automatic frequency control) receiver it is often desired to render the oscillator tank circuit immune from the electrical effect of the frequency control tube network. When the oscillator tank circuit is independent of the frequency control tube network, then the superheterodyne receiver functions in the usual and well known manner. However, in rendering the oscillator tank circuit free of the reactive eiect of the frequency control tube netwo-rk it is essential that the normal, or average, oscillator tank circuit frequency at different settings of the tuning device be undisturbed.

It may, then, be stated that it is one of the main objects of my present invention to provide a device for rendering the oscillator tank circuit of a superheterodyne receiver independent of the electrical effect of a frequency control tube unit of an AFC arrangement, and the device being so constructed and arranged with respect to the control 40 tube .unit and the oscillator tank circuit that the reactance simulated across the tank circuit can be effectively eliminated, when AFC action is not desired, without in any way disturbing the normal, or average, oscillator tank circuit resonant frequency at different settings of the tuning device of the receiver.

Another important object of this invention may be stated to reside in the provision of a switching device in electrical association with the oscillator 50 tank circuit of a superheterodyne receiver and the frequency control tube network of an AFC arrangement, the switch device being constructed and .arranged to prevent the control tube from producing a reactive effect across the oscillator tank circuit when AFC action is undesired, and a UIT Orange, N. J., assignor to America, a corporation of 1937,` serial No. 133,945

(Cl. Z- 40) reactive element being employed in electrical association with the oscillator tank circuit to prevent the normal resonant frequency of the tank circuit from being disturbed by removal of the aforesaid simulated reactance.

Another object of the invention may be stated to reside in the provision of a superheterodyne receiver equipped with an AFC arrangement, and the AFC employing a frequency control tube electrically 4associated with the oscillator tank circuit of the receiver in such a manner that an inductive reactance is simulated across the tank circuit when the AFC is in operation, and a switching element being provided to eliminate said inductive reactance when it is desired to preventthe AFC from acting.

Still other objects of the invention are generally to improve AFC arrangements for receivers of the superheterodyne type, and more especially to provide frequency control arrangements embodying switch devices for controlling the operation thereof, and which switch devices may be incorporated in receivers in an economical .and reliable manner.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown in schematic manner a superheterodynereceiver which employs an AFC arrangement embodying the present invention. In general the receiver may be of the conventional type, and embodies the usual signal collector I which feeds a tunable radio frequency amplier 2. The tunable first detector 3 is supplied with ampliiied signal energy, and oscillations from local oscillator l are also impressed on the detector 3. The intermediate frequency energy output of the latter is amplied by an I. F. amplifier 5, and the output of the latter is demodulated by the usual second detector network. The latter and successive audio stages are omitted, because they are well known to those skilled in the art. Any AVC arrangement may be used to maintain the signal intensity level at the demodulator input circuit substantially uniform.'

The signal circuits 2 and 3 have the rotors of the variable condensers thereof arranged for mechanical uni-control; the rotor of the variable condenser 6 is mechanically coupled to the adjusting means, denoted by the dotted lines 1, for the rotors of the signal circuit condensers. The tank circuit ofthe local oscillator 4 includes the coil 8 shunted by the grounded variable condenser 6. The xed condensers 9 are the well known padders which function to maintain the frequency of the oscillator tank circuit different from that of the signal circuits by a predetermined constant amount throughout the adjustment range of tuning device When the receiver is of the broadcast type and tunable through a range of 500 to 1500 k. c., then the I. F. may have a value chosen from a range of '75 to 480 k. c.

The receiver may be of the multi-range type if desired; further, the oscillator can be of the xedly tuned type as in the case of the second oscillator of a triple detection superheterodyne receiver. To secure accuracy in tuning, as well as to compensate for oscillator drift, there is ernployed an AFC arrangement. The AFC may be of the type shown in application Serial No. 45,413, filed October 17, 1935 by S. W. Seeley. The AFC generally comprises a discriminator I functioning to derive a direct current voltage V(AFC bias) from the I. F. energy; the polarity and magnitude of the bias being dependent on the sense and amount of frequency shift of the I. F. energy from the assigned frequency. The AFC bias is applied through lead II to an electrode of the frequency control tube I2; the latter functions to produce a predetermined reactive effect across tank circuit 8 6.

Since the specific construction of the discriminator network is of little importance in this case, it Will be understood that the network can be of any desired type as long as it is capable of converting a frequency shift in I. F. into a direct current voltage change in polarity and magnitude. For example, the discriminator may comprise oppositely mistuned diode rectifiers as shown by C. Travis in application Serial No. 4,793, led Feb. 14, 1935, or the I. F.-tuned diode rectifiers of the said Seeley application may be used. The circuit details of the frequency control tube and its connections to the oscillator tank circuit will now be described, since the present invention is embodied in that network.

The plate I3 of control tube I2 is connected to the positive terminal (+B) of a direct current source through a series path which includes coil 8; coil |4 (having a magnitude of approximately 4 microhenries); and resistor I5 (of about 2000 ohms). The junction of coil I4 and resistor I5 is connected to ground by condenserV I6 having a value of about 0.912 microfarad. The cathode I1 of control tube I2 is grounded through the usual self-bias resistor-shunt capacity network I8; From the plate side of coil 8 is connected a series path to ground, the series path including resistor |9 and condenser 20. The grid 2`I of control tube I2 is connected to the junction of resistor I9 and condenser through the condenser 22. There is impressed on the grid 2| alternating currentv voltage developed across the path I9 2 by oscillator tank current flow through said path.

Between the plate I3 and grid 2| are disposed two additional grids; one of them is connected to a source of positive voltage and acts as a positive screen grid, while the gridV adjacent to the plate is a suppressor grid which is at cathode potential. The AFC lead II is connected to the grid side of condenser 22 through' the filter resistor 35. By varying the bias of grid 2| the mutual conductance, or gain, of tube I2 is varied. This in turn changes the space current ow to plate I3, and

the current flow through the coil 8. A switch is provided to eliminate the control action of tube I2 when desired; and this elimination of control is secured without disturbing the normal operating frequency of the tank circuit 8 6. The switch comprises connected blades 3| and 32; the junction of the blades is connected to ground through the condenser 33 having a capacity value of about 9.95 microfarad. The contact 34 is connected to the resistor side of condenser 22; the contact 35 is connected to the junction of coils B and I4. VThe switch, when in open position as in the figure, permits the AFC tube to produce across tank circuit 8 3V an inductive effect. When the switch is closed, that is when blades 3| and 32 are in contact with points 34 and 35 respectively, the condenser 33 renders ineective condenser 20 and coil I4.

Since condenser 2f) is essential to the operation of the frequency control action of tank circuit 8 5, it will be seen that rendering ineffective the condenser effectively renders the tank circuit 8 3 independent of frequency control. Briefly, the control action by tube I2 is produced in the following manner. The control circuit proper consists of tube I2, the resistors 39 and I9, and the condensers 22 and 20. A certain alternating voltage, say EB, exists across the tuned circuit 8 6, or across inductor 8. The same voltage exists across resistor IS and condenser 20 in series. If resistor I9 is a high resistance, the current through it is nearly in phase with Ea. The voltage across condenser 20, however, will lag nearly 90 degrees behind Ea. 'I'hat is, the alternating voltage applied to grid 2| of tube I2 lags the plate voltage thereof; but the plate current is in phase with the grid voltage. Therefore, the plate current lags behind the plate voltage nearly 90 degrees. In an inductance of low resistance, the current also lags 90 degrees behind the voltage. Hence, the frequency control tube I2, connected as shown, electrically simulates in shunt across coil 8 an inductance with a small resistance in series with the latter.

This shunt inductance acts to reduce the effective inductance of circuit 8 6; it increases the frequency of oscillation. The AFC bias applied to grid 2| acts to vary the gain of the control tube, and hence the magnitude of the simulated shunt inductance, so as to secure desired oscillator frequency correction in response to a shift in I. F. from the assigned value. The mean bias on grid 2| is so chosen that there will be approximately equal'frequency changes on both sides of the mean. It will now be seen that if condenser 29 is rendered ineffective, through path 34 3| 33, the control tube is incapable Vof producing the shunt inductive effect across coil 8.

Obviously removing the simulated shunt inductance across coil 8 results in increase of the effective inductance of circuit 8 6. Since a normal effectiveinductance for circuit 8 6 is predicated on the coil 8 in shunt with the simulated inductance, the normal frequency of circuit 8 6 will be departed from when the condenser 20 is rendered ineffective. Hence, coil I4 is inserted in series With coil 8 to prevent the removal of the simulated shunt inductance from affecting the effective inductance of circuit 8 6. The magnitude of coil i4 is chosen so as exactly to supply that amount of shunt inductance removed b y rendering ineffective condenser 20.

When the switch is open the coil I4 is in series with coil 8, while the simulated shunt inductance is across coil 8. The effective inductance of circuit 3 6 is, in this condition, just equal to the effective inductance of circuit 8 6 when condenser Z is rendered ineffective, and coil I4 is rendered ineffective by path 35-'-32-33. Hence, the normal operating frequency ofthe oscillator tank circuit, at any setting of condenser 6, will not be affected by adjustment of the frequency control tube switch 3|-35. l

Auxiliary coil I4 would have a -magnitude equal to the difference in eifective reactance of coil 3 with tube I 2 operative and inoperative.

Assume,

L1 is the inductance of coil 8,

L2 is the effective inductance produced by tube l2,

Ls is the inductance of auxiliary coil I4.

if coil it is 4 ,ch and coil 8 has a typical value (for an I. F. of 460 k. c.) of 120 ph, then the simulated inductance in shunt at the normal gain of tube i2 would be 3500 frh.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In combination with a resonant circuit of the type including a coil and a condenser in shunt therewith, said circuit being tuned to a desired `frequency, an electron discharge tube including at least a cathode, a control grid and an anode, means for connecting the anode of said tube to a point on said coil whereby space current of said tube flows through the coil, a reactive path in shunt with said coil for developing alternating voltage from current flowing through said coil, means for impressing alternating voltage developed across said path upon said control grid, the voltage impressed on said control grid being so related in phase with respect to the said current flow through said coil that an inductive reactance is simulated in shunt with said coil, means for varying the gain of said tube to vary the magnitude of said simulated reactance, and additional means for impairing the impression of said alternating voltage on said control grid to an extent suflicient to render the resonant circuit immune from said simulated reactance.

2. In combination with a resonant circuit of the type including a coil and a condenser in shunt therewith, said circuit being tuned to a desired frequency, an electron discharge tube including at least a cathode, a control grid and an anode, means for connecting the anode of said tube to a point on said coil whereby space current of said tube flows through the coil, a reactive path in shunt with said coil for developing alternating voltage from current iiowing through said coil, means for impressing alternating voltage developed across said path upon saidcontrol grid, the voltage impressed on said control grid being so related in phase with respect to the 'said current iiow through said coil that an inductive reactance is simulated in shunt with said coil, means for varying the gain of said tube to vary the magnitude of said simulated reactance, additional means for impairing the impression of said alternating voltage on said control grid thereby to render the resonant circuit immune from said simulated reactance, and said last means comprising a device for rendering ineffective the reactive portion of said reactive path.

3. In combination with a resonant circuit of the type including a coil and a condenser in shunt therewith, said circuit being tuned to a desired frequency, an electron discharge tube including vat least a cathode, a control grid and an anode, means for connecting the anode of said tube to a point on said coil whereby space current of said tube iiows through the coil, a rcactive path in shunt with said coil for developing alternating voltage from currenrt flowing through said coil, means for impressing alternating voltage developed across said path upon said control grid, the voltage impressed on said control grid being so related in phase with respect to the said current flow through said coil that an inductive reactance is simulated in shunt with said coil, means for varying the-gain of said tube to vary the magnitude of said simulated reactance, additional means for impairing the impression of said alternating voltage on said control grid thereby to render the resonant circuit immune from said simulated reactance, said reactive path including a resistor in series with a condenser, and said last means including a device for rendering ineffective said reactive path condenser.

4. In combination with a resonant circuit of the type including a coil and a condenser in shunt therewith, said circuit being tuned to a desired frequency, an electron discharge tube including at least a cathode, a control grid and an anode,

`means for connecting the anode of said tube to a point on said coil whereby space current of said tube flows through the coil, a reactive path in shunt with said coil for developing alternating voltage from current flowing through said coil, means for impressing alternating voltage developed across said path upon said control grid, the voltage impressed on said control grid being so related in phase with respect to the said current flow through said coil that an inductive reactance is simulated in shunt with said coil, means for varying the gain of said tube to vary the magnitude of said simulated reactance, additional means for impairing the impression of said alternating voltage on said control grid thereby to render the resonant circuit immune from said simulated reactance, an auxiliary inductive reactance in series with said resonant circuit coil, said last means being constructed and arranged simultaneously to render ineifective said auxiliary reactance, and the auxiliary reactance having a magnitude such that said resonant frequency is undisturbed when said auxiliary reactance is ineffective and said simulated reactance effect is eliminated.

5. In a superheterodyne receiver of the type including an automatic frequency control circuit, and said automatic frequency control circuit being of the type which includes a frequency control tube having input and output electrodes electrically coupled with the local oscillator tank circuit of the receiver to produce a simulated reactance effect across the tank circuit, a control switch connected to said tank circuit and control tube input electrodes to prevent alternating voltage developed' across said tank circuit from affecting the control Ytube to an extent sufcient to produce said reactance effect, and an auxiliary reactance of the same sign as said simulated reactance electrically associated With said tank circuit to prevent the normal resonant frequency of the tank circuit from being disturbed when said switch is actuated.

6. In a superheterodyne receiver of the type including an automatic frequency control circuit, and said automatic frequency control circuit being of the type which includes a frequency control tube having input and output electrodes electrically coupled with the local oscillator tank circuit of the receiver to produce a simulated Vreactance eiect across the tank circuit, a control switch constructed and arranged to prevent alternating voltage developed across said tank circuit from aiecting said control tube to produce said reactance eiTect, an auxiliary reactance of the same sign as said simulated reactance electrically associated with said tank circuit to prevent the normal resonant frequency of the tank circuit from being disturbed when said switch is actuated, a reactive path in shunt with said tank circuit for developing an alternating current Voltage from current iowing through the tank circuit from said control tube output electrode, means for impressing said alternating voltage on said input electrode, and said switch operating to prevent impression of said alternating voltage on said input electrode.

7. In combination With a resonant circuit of the type including a coil and condenser in shunt therewith, said circuit being tuned to a desired frequency, an electron discharge tube including at least a cathode, a control grid and an anode, means for impressing the output current of said tube on said resonant circuit, a circuit element connected to said resonant circuit whereby the voltage across said circuit element is substantially in quadrature with the voltage across said resonant circuit, said quadrature voltage being applied to the control grid of said electron discharge tube, whereby the effective reactance of the coil of said resonant circuit is changed from the value it would have without said electron tube connections thereto', an additional reactive element connected in the resonant circuit of magnitude equal to the diierence between the effective reactance of the coil of said resonant circuit when the electron tube is operative and when it is inoperative, and switching means for simultaneously rendering said electron tube inoperative and removing from the resonant circuit'the effect of said additional reactive element. DUDLEY E. FOSTER. 

